Pectus excavatum is the most common congenital chest deformity. This defect is characterized by malformed costal cartilages causing a depressed sternum. Surgical correction requires major surgery, hospitalization for pain control, and costs of $60,000 to $80,000. The Magnetic Mini-Mover Procedure (3MP) is an outpatient procedure that uses a magnetically coupled implant to pull the sternum forward and gradually remodel the deformed costal cartilage. In an ongoing FDA-sponsored Phase 1/2 study, the investigators are studying the safety and probable efficacy of the 3MP in 10 patients ages 8-14 years with Pectus Severity Index >3.50 (normal = 2.56). Interim analysis demonstrates that the 3MP is a safe, outpatient, minimally-invasive, and cost-effective treatment for pectus excavatum, that brace-wear compliance was 10-21 hours/day (mean 16.8 h/day), and that the pectus deformity improved more rapidly in prepubertal patients. In addition, this initial Phase 1/2 trial has led to minor improvements in the titanium-encased magnetic implant (Magnimplant) and major improvements in the external magnet brace (Magnatract). The second generation Magnatract allows the patient to adjust the force applied, measures the force by an indwelling pressure sensor, logs force and temperature every 10 minutes, and downloads the data securely over the Internet. Consultation with the FDA suggested that further data on safety and probable efficacy and demonstration that the procedure can be safely used at other centers would allow application for a Humanitarian Device Exemption (HDE). The investigators will extend the findings of the initial trial in a Phase 3 study that incorporates the improvements from the first trial in the second generation Magnetic Mini-Mover Procedure. The investigators propose enrolling 15 additional patients ages 8-12 with Pectus Severity Index >3.50 at three centers.